Search Results (5)

Coastal waters, particularly in micro-tidal estuaries, are highly vulnerable to water quality changes driven by salinity intrusion. Effective coastal water management requires a detailed understanding of the dynamic processes in estuaries to mitigate the effects of salinity intrusion. This study focuses on the Brisbane River estuary (BRE), Australia, to investigate salinity intrusion and flushing time under varying freshwater inflows. A coupled MIKE 21 FM hydrodynamic (HD) and transport (TR) model was used to assess salinity transport during a neap–spring tidal cycle. The 2D model was calibrated and validated for the 2008 and 2011 flow events using field data on water levels and salinity. Results show an increase in tidal amplitude landward under low river flow conditions, while tidal damping was observed under higher river discharge, reducing the BRE salinity during spring tides. This study found that salinity intrusion is highly sensitive to freshwater availability, with river discharges of 150 m³/s and 175 m³/s identified as critical thresholds to maintain salinity levels below 1 PSU at the estuary mouth during ebb and flood tides, respectively. Flushing time analysis indicates that the BRE takes approximately 302 h to completely displace saline water when the river discharge is optimized at 150 m³/s. Modifying the BRE estuary mouth—through channel widening or deepening—enhanced the flushing process, significantly reducing salinity levels. This study demonstrates that optimizing freshwater discharge and modifying the estuary mouth can provide effective solutions for managing salinity intrusion in micro-tidal estuaries. Full article

Improving water quality is imperative for many sea bays, including Laizhou Bay, China, to achieve sustainable marine development. In 2010, two 17.3 km long sand-retaining embankments were built in southwestern Laizhou Bay, which weakened the nearshore hydrodynamics and seriously impacted the water quality. To mitigate this issue, this study proposes connecting the two rivers on both sides of the embankments to improve the hydrodynamics and thus water exchange in the sea bay. The effectiveness was examined with a numerical model using Mike 21, which was validated for both tidal current velocity and direction at six monitoring locations in the sea bay. The results show that over 53% of the core research area displays an increase (0.0–0.4 m/s) in tidal current speed after the connection, primarily in and around the Haihengwei Fishing Port. Meanwhile, the Eulerian residual currents in the Haihengwei Fishing Port, Mi River estuary and Bailang River estuary become substantially larger (with a maximum increase of 0.16 m/s). In addition, the net transport distance of particles released near the connection increases by up to 39.89 km in one month. Overall, this case study demonstrates that connecting rivers next to a harbor can effectively improve hydrodynamics and thus improve water quality in the bay. Full article

Domestic urban streams face insufficient base flow and consequently become dry streams in drought season, and vulnerable to water quality deterioration and ecological impairment, due to contaminants introduced from the urban pollutants. Many efforts are being made to improve the natural flow by actively enforcing restoration projects of urban streams. Gulpocheon is a national stream flowing through Incheon-si and Gimpo-si. As of March 2019, the reclaimed wastewater or the ozone-processed Gulpo treated sewage has formed the upper part of Gulpocheon. This study aimed to analyze the improvement in water quality of Gulpocheon before and after supplying the reclaimed water by collecting the water quality data of the target area. Before and after providing the base flow, the water quality was analyzed using the two-dimensional numerical analysis model, i.e., MIKE 21 FM. The water quality one year before and after supplying the reclaimed water was compared, with a focus on DO, BOD, TN, and TP; they are used as water quality standards for stream water. The concentration of DO at all spots of Gulpocheon increased on average. The concentration of BOD, TN, and TP water quality parameters decreased, indicating water quality improvement. In addition, accurate water quality assessment is possible using MIKE 21 FM model simulation for urban stream analysis. Full article

Bridge piers on river channels can cause obstacles for flood flow by reducing the cross-sectional area and inducing local eddy currents and high flow velocities, which may destroy hydraulic structures. A two-dimensional numerical model was used to investigate the effects of bridge piers on river flood hazards in the Jialing River, China. For the modeling, Mike 21 FM was used, which is an unstructured mesh and finite volume model that solves the shallow water equations. The numerical model was validated with collected historical flood traces, and sensitivity analyses identified the effects of the Manning coefficient and the dependence on the grid size. The influence of backwater effects on the flow field was analyzed by comparing numerical results with and without piers. The results showed that the most significant impacts were caused by the Fengxian Bridge. The maximum water level rise was about 1 m and the maximum velocity near this bridge decreased by 22.77% for a 10-year flood. We found that the top elevations of planned levees near the bridges must be increased by 0.15–0.36 m. The influence of bridge piers on the flood velocity field is more complex. These findings will help flood hazard management in this river and provides a reference for similar projects. Full article

The pollutant residence time and transport trajectory in Tieshangang Bay are considered to have significant effects on deteriorating water quality. To understand the pollutant transport behaviors in Tieshangang Bay, we developed a combination model (MIKE 21 FM) of the hydrodynamic module and particle tracking module. Simulation results suggest that the water velocities in the west and east troughs (near the entrance of the bay) are distinctly higher than any other areas. Meanwhile, small semi-enclosed bays adjacent to the shoreline could affect local water flow patterns, thereby causing gyres within them. The residence time of pollutants in Tieshangang Bay is significantly affected by seasonal variations (i.e., the residence time of pollutants in Tieshangang Bay in winter is less than that in summer). The results of transport trajectory simulations reveal that the bay head is a slow flushing zone, while the entrance of the bay (west trough) can be identified as a fast flushing zone. Full article